This invention relates to a process for the preparation of isocyanates.
It is known that the reaction of carboxyl amides with a hypohalide in an alkaline aqueous, or alkaline alcoholic medium will lead to the intermediate formation of an N-halogen amide and subsequently, through thermal degradation (Hofmann degradation), to an isocyanate. However, the foregoing synthesis is not suitable for the preparation of isocyantes, because the highly reactive isocyanates immediately continue to react, either with water to form an amine, or with alcohol to form a urethane. Additionally, part of the resulting isocyante reacts with unconverted N-halogen amide, to form a halo-acyl urea.
U.S. Pat. No. 3,483,242 teaches a process by which one starts with an N-halogen amide and heats the same in a stream of inert gas to temperatures ranging between the melting point of the N-halogen amide and 400.degree. C. The purpose of the stream of inert gas, the velocity of which is between 25 and 1000 centimeters per minute, is to dilute the mixture to such an extent that the reaction products cannot react with one another. In addition, the stream of inert gas moves the reaction products to a condenser, where they are condensed and separated. In order to increase the yield, the reaction is performed in a solvent, such as aliphatic and aromatic nitro compounds, aromatic hydrocarbons, tertiary amines, halogenated hydrocarbons, and glycol ethers, preferably aprotic solvents, and particularly chloroform.
In the aforementioned process, the use of a base is not considered necessary, but in some cases a base is indicated to bring about an increase in yield. Inorganic bases, such as sodium hydroxide, calcium oxide, sodium carbonate, or aluminum oxide, are exclusively recommended. The process may be performed above the melting poing of the halogen amide being used. However, since the halogen amides decompose below their melting point, with formation of undesirable by-products, the attainable isocyanate yields are low. With phenyl isocyanate the yield is a maximum of 39.9%, for benzyl isocyanate 8.6%, for para-methoxyphenyl isocyanate 17%, and for cyclohexyl isocyanate 30%, of theoretical.
The object of the present invention is thus to prepare numerous aliphatic, alkyl aromatic, and cycloaliphatic mono-, bis and tris-isocyanates with substantially higher yields than those obtainable with the processes of prior art.